1. Field of the Invention
This invention relates generally to thermal regenerators and more particularly to a thermal regenerator that uses thin, planar sheets of material of sufficient thermal conductivity to form the heat transfer surfaces of the regenerator.
2. Description of the Related Art
Many devices, and Stirling cycle machines in particular, include a thermal regenerator to which thermal energy is transferred from a flowing fluid, and from which thermal energy is transferred to the fluid. Regenerators are normally made with large surface area structures, such as wool, foils or spheres, made of metal, such as stainless steel.
In a Stirling cycle engine, for example, a working gas is moved between a warmer space and a cooler space by a reciprocating displacer to drive a reciprocating piston. The gas is heated during one part of the cycle, and cooled during another part. When the warm gas is being transported from the warmer space, it flows through a regenerator, and thermal energy is transferred to the regenerator by convection, i.e., the impingement of heated gas molecules on the regenerator's surfaces. The regenerator is warmed and the gas is cooled when thermal energy is transferred to the regenerator as the gas flows through the regenerator to the cooler space.
Once the gas has been cooled in the cooler space, it is driven again through the regenerator; ordinarily in the opposite direction as when the gas was driven from the warmer space. The cooler gas flowing through the regenerator is warmed by the same convection mechanism by which the gas warmed the regenerator: impingement of gas molecules on the regenerator's surfaces. Regenerators therefore improve the efficiency of the Stirling cycle engine because the gas enters the heated end pre-warmed, and gas enters the cooler end pre-cooled. Of course, regenerators improve the efficiency of many machines other than Stirling cycle machines.
In conventional regenerators, there must be a substantial amount of contact between the flowing fluid molecules and the surfaces of the regenerator in order for substantial heat transfer to occur. One type of regenerator used in Stirling cycle machines uses a long thin strip of metal, such as stainless steel, that is wound up in a roll and placed in a chamber through which gas flows longitudinally of the roll. Each layer of the metal has a space or gap between it and the next adjacent layer for fluid to pass through.
Even though it is desirable to have uniform spacing of the layers of a regenerator, it is often difficult, in practice, to achieve such uniformity of spacing. A temperature differential between the heated end and the cooled end may cause buckling, which varies the gap sizes. Additionally, the flow of fluid through a wound regenerator cannot distribute evenly radially, which can cause areas with substantially more flow to expand or contract the metal more than areas with less flow. All of these problems result in high fluid flow rates through larger gaps, and low flow rates through smaller gaps. Non-uniform flow is disadvantageous, because large gaps permit some gas flowing through the regenerator to make poor contact with the surfaces with which thermal transfer should take place. Furthermore, the pressure drop that is critical to the class of machines referred to as free-piston machines is often compromised with conventional regenerators, thereby resulting in unanticipated dynamic motion of the moving parts.
There is therefore a need for a regenerator that maintains substantially uniform spacing throughout the entire region of the regenerator through which fluid flows.